Pumping over refers to a symptom of an improperly configured central heating system in which water is discharged into the feed and expansion cistern via the expansion pipe while the pump is running. The cause is usually either:
- The pump has been installed in a sub-optimal location on the system
- The pump has been installed the wrong way, and is directing water the wrong way around the system
- The pump’s speed is too high
While seemingly inconsequential at first, pumping over can have devastating effects on a central heating system in the long term. When the system is pumping over, the feed and expansion cistern and the expansion pipe together form the perfect aerator, enriching the water with oxygen like an aerator in a fish tank. The oxygen-rich water accelerates the production of oxides within the system, and eventually cripples it via corrosion: heat exchangers get blocked up, and radiators develop leaks due to internal rusting. If left uncorrected, the cost of repairing the damage to the central heating system and damage to the property can run into hundreds or even thousands of pounds. Cleansing, powerflushing, or adding inhibitor to the system will not resolve the issue, since the cause most likely relates to how water is distributed around the system.